SMC X-2 outburst reaches 1E38 erg/s
ATel #15500; M. J. Coe (Soton), J. A. Kennea (PSU), P. A. Evans (Leicester), D. A.H. Buckley (SAAO), L. J. Townsend (SALT), I. Monageng (SAAO)
on 5 Jul 2022; 09:00 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)
Subjects: Optical, X-ray, Binary, Neutron Star, Transient
More than 3 weeks after its first detection by S-CUBED (ATel #15434) SMC X-2 continues to brighten in the X-rays. The most recent observations by Swift XRT on 3 July 2022 indicate it has now reached 1E38 erg/s and still getting brighter. Our previous quoted value in ATel #15434 of 3E35 erg/s was incorrect, it should have been 2E37 erg/s. Hence it is exhibiting an unusually slow brightening for what appears to be a Type II outburst (its binary period is 18.6d). Normally a Type II outburst from a Be/X-ray binary would rapidly peak in just a few days.
An optical red spectrum was obtained using SALT on 3 July 2022 revealed the H alpha line in emission, characteristic of these Be stars in such binary systems. The measured H alpha equivalent width was 5A, almost identical to a previous measurement taken at the SAAO 1.9m telescope in November 2013 when the source was in quiescence. This small, unchanged value indicates little or no recent growth in the circumstellar disk, yet another puzzling feature of this outburst.
Swift XRT will continue to monitor SMC X-2 throughout this unusual extended outburst.
